1. Field of the Invention
The present invention relates to trench shields and manhole shields.
2. Description of the Prior Art
Trench shields and manhole shields are designed to protect workers in deep excavations, e.g. when laying pipe, from a slope failure of an exposed earth wall. In laying pipe, a pipe joint is laid and the space within the trench or manhole shield is partially backfilled and compacted. The shield is then pulled forward in order to lay the next section of pipe. Frictional forces resulting from the pressures of the soil collapsing against the exterior walls of the shield and the material backfilled and compressed against the interior walls of the shield exist. These forces must be overcome in order to advance the shield in a trench or extract it.
Previous patents, for example U.S. Pat. No. 2,922,283, issued on Jan. 26, 1960 to F. S. Porter, U.S. Pat. No. 3,089,310, issued on May 14, 1963 to E. Torti, U.S. Pat. No. 4,019,330, issued on Apr. 26, 1977 to James L. Griswold, U.S. Pat. No. 4,752,157, issued on Jun. 21, 1988 to Ernst-Friedrich Ischebeck et al., U.S. Pat. No. 4,993,880, issued on Feb. 19, 1991 to John Collins, U.S. Pat. No. 5,232,312, issued on Aug. 3, 1993 to Charles B. Jennings et al., U.S. Pat. No. 5,277,522, issued on Jan. 11, 1994 to Joseph F. Pertz, U.S. Pat. No. 5,290,129, issued on Mar. 1, 1994 to Kenneth G. Rody et al., U.S. Pat. No. 5,310,290, issued on May 10, 1994 to Dennis I. Spencer, U.S. Pat. No. 5,503,504, issued on Apr. 2, 1996 to Wilhelm Hess et al., and U.S. Pat. No. 5,527,137, issued on Jun. 18, 1996 to Dennis I. Spencer, Great Britain Patent Nos. 2,094,373, published on Sep. 15, 1982, 2,115,453, published on Sep. 7, 1983, and 2,171,735, published on Jan. 22, 1986, French Patent No. 2,615,218, issued on Nov. 18, 1988, and German Patent No. 3,820,625, issued on Jan. 5, 1989, teach shoring devices and machines. Further, the Efficiency Production, Inc. brochure, published in 1991, teaches trench shields.
These prior trench shields all suffer from the above-noted disadvantage, namely that frictional forces must be overcome, e.g. by the use of heavy equipment. The shields of the instant invention have tapered side shields, thereby resulting in reduced frictional forces as they are advanced forward in the trench or extracted from the trench.